1. Field of the Invention:
The present invention relates to an apparatus for chemical vapor deposition, and more particularly to an apparatus for chemical vapor deposition for forming films on surfaces of semiconductor substrates placed along a conveying path through chemical vapor deposition reaction.
2. Description of the Related Art:
FIG. 1A is a schematic diagram of a conventional apparatus for chemical vapor deposition (hereafter referred to as the atmospheric pressure CVD apparatus) for forming thin films under atmospheric pressure. In FIG. 1A, semiconductor substrates, e.g. silicon wafers 2, are placed on conveying trays 1 and are conveyed by conveying means 30. The conveying means 30 comprises a tray conveyance driving section 3 and a tray conveying chain 4 and to conveying the silicon wafers 2 in the direction of the arrow 5. A preheater 6, a main heater 7, and a postheater 8 are disposed consecutively in the direction of conveyance of the silicon wafers 2 below the wafer conveying trays 1. Gas dispersing heads 9 for blowing a reaction gas for causing a chemical vapor deposition reaction onto the silicon wafers 2 disposed immediately therebelow are provided above the wafer conveying trays 1. A reaction gas 10 is introduced into the gas dispersing heads 9.
A description will now be given of a case in which boro-silicate glass films (hereafter referred to as the BSG films) are formed on the silicon wafers 2 as the semiconductor substrates by using this atmospheric CVD apparatus. Incidentally, the BSG films are used as interlayer insulating films for semiconductor devices.
First, the silicon wafers 2 are placed on the wafer conveying trays 1. The wafer conveying trays 1 are then conveyed in the direction of the arrow 5 by the conveying means 30 composed of the tray conveyance driving section 3 and the tray conveying chain 4. A preheater 6, a main heater 7, and a postheater 8 are provided in a central portion of the atmospheric pressure CVD apparatus as heating means for a thermal chemical vapor deposition reaction to take place. These heaters are provided to make uniform the temperature distribution in film forming regions of the silicon wafers 2 and are so arranged to control each of the regions independently. When BSG films are formed, the heating temperature for the silicon wafers is preferably 330.degree.-450.degree. C. The preheater 6 is used to preheat the wafer conveying trays 1 and the silicon wafers 2 to the vicinity of a desired temperature for forming films and serves to stabilize the film forming temperature in the region of the main heater 7. In addition, the postheater 8 serves to gradually cool the wafer conveying trays 1 and the silicon wafers 2 after the film formation. A reaction gas 10, which consists of SiH.sub.4, B.sub.2 H.sub.6, or O.sub.2 diluted by an inert gas such as N.sub.2, for forming BSG films, as well as N.sub.2 as a carrier gas, is introduced into the gas dispersing heads 9 provided above the main heaters 7 so as to form BSG films. After these component gases are mixed in the gas dispersing heads 9, they are blown onto the heated silicon wafers 2 placed on the wafer conveying trays 1 as a reaction gas 11.
Consequently, thermal chemical vapor reaction takes place on the surfaces of the silicon wafers 2, as shown below, and Si oxide films containing B.sub.2 O.sub.3, i.e., BSG films, are formed on the surfaces of the silicon wafers 2. EQU SiH.sub.4 +2O.sub.2 .fwdarw.SiO.sub.2 +2H.sub.2 O EQU B.sub.2 H.sub.6 +3O.sub.2 .fwdarw.B.sub.2 O.sub.3 +3H.sub.2 O
In FIG. 1A, a plurality of gas dispersing heads 9 are used. The length (denoted by reference numeral 12 in FIG. 1B) of the gas dispersing head 9 in the direction 5 of conveyance of the trays cannot be made very large to ensure that the mixed reaction gas 11 will flow uniformly onto the silicon wafers 2. For this reason, individual regions 12 for film formation are relatively short. Accordingly, since a thermal chemical vapor reaction takes place, as shown in the above formulae, if the film forming temperature and the amount of the reaction gas supplied are fixed, the fact that the film forming regions 12 are short means that the film formation rate cannot be increased. For that reason, as shown in FIG. 1A, the plurality of gas dispersing heads 9 (two in the case of FIG. 1A) are provided to substantially improve the film formation rate.
Since the conventional atmospheric pressure CVD apparatus is arranged as described above, when the reaction gas 11 is not allowed to flow from the gas dispersing heads 9, the distribution of the surface temperature of the silicon wafers can be made uniform in the vicinity of the film forming regions 12, as shown by the broken line 13 in FIG. 1B, by controlling the three heaters 6, 7, 8. Incidentally, FIG. 1B illustrates the relationships between a distance of the atmospheric pressure CVD apparatus shown in FIG. 1B in the direction of conveyance of the trays and the surface temperature of the wafers 2. However, since the reaction gas 11 is allowed to flow from the gas dispersing heads 9 toward the silicon wafers 2, the gas in an amount exceeding 10 l/min is blown locally onto the surfaces of the silicon wafers 2. Consequently, the surfaces of the silicon wafers 2 are cooled, and the temperature of the silicon wafers 2 immediately below the gas dispersing heads 9 becomes low. For instance, when the flow rate of the reaction gas 11 is 20 l/min, the surface temperature of the silicon wafers 2 immediately below the gas dispersing heads 9 drops by as much as about 20.degree. C. For this reason, this presents a problem when the quality of the films such as the BSG films are substantially affected by the film formation temperature. Namely, the density of boron in the BSG films varies substantially depending on the film formation temperature, and when the film formation temperature is high, the density of boron in the films tends to decline. This is attributable to the fact that B.sub.2 H.sub.6 contained in the mixed reaction gas 11 injected from the gas dispersing heads 9 reacts with O.sub.2 in the vapor, so that the amount of B.sub.2 H.sub.6 reaching the vicinity of the silicon wafers 2 is reduced as a result. Consequently, the thickness-wise distribution of the boron density in the BSG films formed by the conventional atmospheric pressure CVD apparatus shown in FIG. 1A has two bumps, as shown in FIG. 2A. FIG. 2B is a cross-sectional view of a silicon wafer 51 on which a BSG film 56 has been formed, while FIG. 2A is a graph illustrating the distribution of the boron density along the line A-A' in the thickness-wise direction of the BSG film 56 shown in FIG. 2B. Thus, the BSG film whose distribution of the boron density in the thickness-wise direction is not uniform often presents a problem when contact holes or the like are to be provided by wet etching.
FIGS. 3A and 3B are enlarged partly cross-sectional views of silicon wafers to illustrate this problem. The etching rate of the BSG films in a hydrofluoric acid-based solution depends on the boron density, and a higher boron density results in a lower etching rate. For this reason, when the distribution of the boron density in the thickness-wise direction of the BSG film 56 is uniform, if etching is performed with a hydrofluoric acid using a photoresist 54 as a mask, an isotropic etching configuration 55 is obtained, as shown in FIG. 3A. However, in the case of the BSG film 56 having the two bumps, as shown in FIG. 2A, since the etching rate with respect to the hydrofluoric acid-based solution in the thickness-wise direction is not constant, an abnormal configuration 57 is obtained due to etching, as shown in FIG. 3B. Consequently, when a metal wiring 58 is formed on the portion having this abnormal configuration 57 after removal of the photoresist 54, a discontinuity may result (at the portion indicated by reference numeral 59), and the step coverage of this portion is poor.